Next-generation wireless communication system is required to transmit high-quality and high-capacity multimedia data at high speed under limited radio resources. In order to accomplish those requirements, inter-symbol interference and frequency selective fading need to be overcome while maximizing spectral efficiency.
To improve the performance of a wireless communication system, there has emerged a closed-loop transmission scheme employing channel quality information between a transmitter and a receiver. An adaptive modulation and coding (AMC) scheme is a technology where modulation and coding scheme is adjusted based on the channel quality information in order to increase link performance. The channel quality information can also be used by a transmitter in pre-processing data so as to reduce complexity of equalization process in a receiver.
In general, two kinds of methods are used in order to transmit channel quality information to a transmitter. The first method is based on feedback information transmitted from a receiver. The receiver measures channel quality and configures the channel quality as feedback information. A transmitter that has received the feedback information performs closed-loop transmission. For example, the feedback information can be the quantized form of measured channel quality.
The second method employs channel sounding. In general, channel sounding is based on reciprocity in a time division duplex (TDD) system. TDD is a method in which uplink transmission and downlink transmission share the same frequency, but occur at different times. In the TDD system, uplink channel and downlink channel are reciprocal. This is because, at a specific time-frequency, the downlink channel and the uplink channel are identical. Accordingly, channel quality of the uplink channel can be obtained from that of the downlink channel. A receiver sends a sounding signal to a transmitter through the uplink channel. The transmitter estimates channel quality of the downlink channel and the uplink channel from the sounding signal. The transmitter performs downlink transmission base on the estimated channel quality of the downlink channel.
Frequency division duplex (FDD) is a method in which uplink transmission and downlink transmission use different frequencies. In FDD, downlink channel and uplink channel are not reciprocal. Accordingly, channel quality of the downlink channel, which is obtained from feedback information, cannot be used as channel quality of the uplink channel.
To enable a base station to perform frequency-selective scheduling on uplink radio resources, the base station needs to know uplink channel quality. A user equipment uses a sounding signal for the base station to measure the uplink channel quality.
If a user equipment located at the edge of a cell transmits a sounding signal over the overall bandwidth, severe interference can be caused on neighboring cells. Further, the sounding signal over the overall bandwidth may function as transmission overhead.
In a FDD system, there is a need for a method for transmitting a sounding signal.